For vintage machines that have the kind of foot pedal that opens and closes like clamshell, you can experiment with stuffing a sponge into the clamshell to prevent it going down all the way (and sewing at the fastest speed). I think that's what you ran across with the foam idea. With that kind of foot pedal, it's an easy way to slow down a machine so a child can sew more slowly.
I grew up sewing on a vintage Singer that had the knee control. In my opinion, it was actually much easier to use a knee control than it is to use a foot control (and nicer to the body too!).